1. The present invention relates to a connector for a capacitor microphone and in particular to a shielding structure and a shielding method of such a connector.
2. Description of the Related Art
Capacitor microphones have high impedance microphone units and thus an impedance converter including a field-effect transistor (FET) is used therein.
In tie pin microphones and gooseneck microphones, a microphone is made less noticeable with the following structure. An impedance converter is incorporated in a microphone unit. A low-cut circuit and an output circuit axe stored in a circuit storage provided separately with the microphone unit, and the microphone unit and the circuit storage are connected to each other via, a dedicated microphone cable. The microphone unit converts sound into an electric signal. The sound signal thus made is transmitted to the circuit storage to be output from the output circuit therein. The circuit storage incorporating the low-cut circuit and the output circuit is referred to as a power module.
The dedicated microphone cable connecting the microphone unit to the power module is a two-core shielding cable formed of: a power wire through which power is supplied to the capacitor microphone; a signal wire through which a sound signal output from the impedance converter is fed to the power module; and a shielding cable for electric shielding and grounding the wires.
The sound signal, which is transmitted through the dedicated microphone cable in ah unbalanced state, is vulnerable to external noise, i.e., is easily affected by external electromagnetic, waves. Specifically, electro-magnetic waves reaching the dedicated microphone cable from the exterior enters the microphone unit or the power module through the microphone cable. Then, the electromagnetic waves are detected by a semiconductor element forming the microphone unit or the power module to be mixed into the sound signal as noise.
An output from the microphone is output, from the power module through the balanced shielding cable. Still, if strong electromagnetic waves are, applied to the microphone or the output cable of the microphone, high frequency current enters the microphone through the microphone cable and via a microphone connector and is demodulated in the impedance converter to be output from the microphone as noise in an audible frequency level.
The microphone cable can be attached and detached to and from the microphone via a threepin type microphone connector (a connecter specified in EIAJ RC-5236 “Latch Lock Type Round Connector for Acoustic Equipment”. First to third pins of the threepin type microphone connector are generally used for grounding, a hot side of a signal, and a cold side of a signal, respectively.
In a connector mounted oh a general microphone cable, core wires and a shielding cable of the cable are directly connected to male and female parts, of the connector that are in contact with each other by means of soldering and the like, and the first pin is connected to a housing of the connector made of metal through a lead wire. Accordingly, impedance for high frequency waves is present between the shield of the microphone cable and the connector housing allowing the high frequency current to enter therethrough.
FIG. 4 exemplary illustrates a conventional connector of a capacitor microphone used for a dedicated microphone cable. In FIG. 4, the reference numeral 10 denotes the connector. The connector 10 is a female connector and is electrically connected to a male connector of a microphone not illustrated when the male connector of the microphone is inserted therein. The connector 10 is of a threepin type and includes: three pins that fit the male connector of the microphone; and terminal plates that are electrically integrated with the pins and protrude from the rear end of the connector 10. Core wires 23 and 24 and a shielding cable 22 on one side of the microphone cable 20 are connected to the respective terminal plates by means of soldering. The microphone cable 20 is passed through an insulating sleeve 60, a sleeve 70, and a bush 40 in this order at its outer periphery.
The insulating sleeve 60 has an outer diameter substantially the same as that of the connector 10 and covers the connection, portion between one end of the microphone cable 20 and the connector 10 to protect the connection portion and prevent short-circuit thereat. The sleeve 70 includes a cylindrical part 71 having an inner diameter substantially the same as the outer diameter of the sleeve 60 and covers the connection portion between one end of the microphone cable 20 and the connector 10 with a certain space provided therearound, and claws 72 for compressing the outer peripheral surface of the microphone cable 20, the surface formed of an insulating cover, to allow the sleeve 70 to be integrally connected to the microphone cable 20. The bush 40 includes a root portion 41 of a tapered shape having the inner diameter slightly larger than the outer diameter of the microphone cable 20 and a cover 42 that has a diameter larger than that of the root section 41 and can cover the sleeve 70.
The connector 10 is fitted in a cylindrical connector housing 50 that is long enough to cover the connector 10, the insulating sleeve 60, and the cylindrical part 71 of the sleeve 70. Outer periphery on the rear side of the connector housing 50 fits the inner periphery on the front side of the bush 40.
FIGS. 4A to 4C illustrate an assembling sequence. As illustrated in FIG. 4A, the connector 10 and the microphone cable 20 are connected with each other with the terminal plates of the connector 10 soldered to the respective wires and the cable of the microphone cable 20. The microphone cable 20 is so passed through the insulating sleeve 60 and the sleeve 70 before or after the soldering that the front end of the insulating sleeve 60 abuts the rear end of the connector 10, and the front end of the cylindrical part 71 of the sleeve 70 abuts the rear end of the insulating sleeve 60, whereas the claws 72 of the sleeve 70 is compressed to connect the sleeve 70 to the microphone cable 20 as illustrated in FIG. 4B. Then, the outer periphery on the rear side of the connector housing 50 covering the connecter 10, the insulating sleeve 60, and the sleeve 70 is fitted to the inner periphery on the front side of the bush 40, thereby integrating the connector housing 50 and the bush 40. Thus, the connector is formed with the connector hosing 50 and the bush 40 as well as the connector 10, the insulating sleeve 60, the sleeve 70, and the microphone cable 20 integrally connected.
In the conventional example illustrated in FIGS. 4A to 4C, a shielding structure is provided to the connector portion by connecting the shielding cable 22 to the sleeve 70 that fixes the microphone cable 20 to internally connect the shielding cable 22 to the connector housing 50. The shielding cable 22 of the microphone cable 20 and the connector housing 50 are electrically discontinuous because the insulating sleeve 60 and the sleeve 70 are provided between the microphone cable 20 and the connector housing 50. Unfortunately, the discontinuous portion serves as a hole (an opening) for external high-frequency wave and electromagnetic waves passes therethrough.
FIG. 5 illustrates a shielding structure for a microphone cable that is proposed to solve the problem the conventional example illustrated in FIGS. 4A to 4C has. In FIG. 5, one end of the shielding cable 22 of the microphone cable 20 covering the core wires is folded in the opposite direction to be placed on a sheath of the microphone cable 20. This folded portion 21 is passed through a small-diameter cylindrical part 81 of a sleeve 80. The sleeve 80 and the shielding cable 22 are electrically connected and the sleeve 80 and the microphone cable 20 are connected by compressing the small-diameter cylindrical part 81. The sleeve 80 also includes a large-diameter cylindrical part 83 having an outer diameter substantially the same as the inner diameter of the connector housing 50 at the rear end. The connector housing 50 fits the outer periphery of the large-diameter cylindrical part 83 to make the sleeve 80 electrically connected to the connector housing 50.
The conventional example illustrated in FIG. 5 aims to provide higher shielding effect compared with that provided by the conventional example illustrated in FIG. 4 by providing continuous shielding structure by electrically connecting the sleeve 80 and the connector housing 50. Unfortunately, the connection can be no more than a point contact and sufficient shielding effect cannot be provided because the connector housing 50 and the sleeve 80 only have their opposing end surfaces pressed against each other.
FIG. 6 illustrates a shielding structure of a microphone connector disclosed in Japanese Patent Application Publication No. 2006-67165. In FIG. 6, a structure is proposed in which the connector housing 50 and the bush 40 are integrated by fitting the outer periphery oh the rear side of the connector housing 50 covering the connector 10, the insulating sleeve 60, and a large-diameter cylindrical part 31 of a compression sleeve 30 to the inner periphery on the front side of the bush 40. In the connector with such a structure, the integral connection of the connector housing 50 and the bush 40 is accompanied by the integral connection between the connector 10, the insulating sleeve 60, the compression sleeve 30 and the microphone cable 20. In this conventional example, the connection portion of the connector 10 and the microphone cable 20 is covered by the large-diameter cylindrical part 31 of the compression sleeve 30. A small-diameter cylindrical part 32 of the compression sleeve 30 is fitted to the outer peripheral side of the folded portion of the shielding cable 22 at the end of the microphone cable 20 and is compressed to be electrically connected to the shielding cable 22 of the microphone cable 20. The large-diameter cylindrical part 31 of the compression sleeve 30 is fitted to the connector housing 50. All things considered, the connection portion between the connector 10 and the microphone cable 20 is continuously shielded from the shielding cable 22 of the microphone cable 20 to the connector housing 50. Thus, the shielding effect is increased.
A structure is proposed with an invention disclosed in Japanese Patent Application Publication No. 2007-300598 in which, in addition to the structure described in Japanese Patent Application Publication No. 2006-67165 illustrated in FIG. 6, the large-diameter cylindrical part 31 of the compression sleeve 30 and the connector housing 50 are fitted to each other with the sleeve 70 that is conductive and is made of an elastic material and having a cross-sectional, shape of a wave form provided therebetween. The sleeve 70 that is conductive electrically connects the compression sleeve 30 and the connector housing 50 integrally at multiple points. Furthermore, a structure is proposed in which the shield is galvanically separated by using a capacitor sleeve having a capacitor structure in stead of the sleeve 70 that is conductive.
In the inventions disclosed in Japanese Patent Application Publication No. 2006-67165 and Japanese Patent Application Publication No. 2007-300598, the shield is formed by connecting the outer periphery of the compression sleeve 30 to the inner periphery of the connector housing 50. Therefore, electro-magnetic waves may enter the connector housing 50 through gaps between the connector housing 50 and the bush 40 and between the bush 40 and the compression sleeve 30 as illustrated in FIG. 6 with arrows A. Electro-magnetic waves can enter through whatever gap formed in an apparatus as illustrated with the arrows A like liquid entering through a gap. The entrance of electro-magnetic waves leads to the generation of noise in the microphone.
Less attention being paid on shielding at the connector as described above is resulting in the entrance of electro-magnetic waves through the connection portion to have noise mixed into a sound signal.
Due to recent spread of cell phones, electro-magnetic waves of a high frequency exist everywhere in our daily lives. Thus, chances of a high frequency signal entering the microphone cable through its connector to have noise mixed into a sound signal is increasing. Capacitor microphones are especially vulnerable to the high frequency signal from a cell phone used nearby and the high frequency signal entering through the connection portion turns into noise.